This invention relates generally to structural sandwich laminates in which a core has facing skins bonded thereto, and in particular to a laminate whose core is formed by an expanded wood structure to provide a light-weight laminate of high strength.
An expanded metal sheet is formed by slitting a sheet of metal and then stretching the sheet into a lattice. The term expanded plastic is applicable to a cellular plastic structure created by introducing gas into the molten plastic. As used herein, the term expanded wood refers to a cellular structure formed by an array of wood strips having a periodic wave formation, the wavy strips being out of phase with each other and being joined together to define open-ended cells.
While the invention has particular advantages in the context of end grain balsa wood, it is also applicable to other forms of wood fabricated to create an expanded wood structure.
Balsa has outstanding properties unique in the field of lumber, for on the average it weighs less than nine pounds per cubic foot, this being 40% less than the lightest North American species. Its cell structure affords a combination of high rigidity and compressive and tensile strength that is superior to any composite or synthetic material of equal or higher density. Balsa is dimensionally stable and may be processed by standard woodworking techniques.
It is known that end grain balsa wood is capable of supporting far greater loads than flat-grain material of the same density and that low-density balsa in the end-grain direction will support greater loads than flat grained material of higher density. The cellular structure of balsa is such that the number of cells per cubic foot is extremely high, the wall thickness of each cell being quite thin. The cells are effectively independent of each other, each cell being comparable to an independent column or fiber. The fibers are substantially parallel to each other.
Structural sandwich laminates can be created by bonding thin facings or skins to balsa wood panels which function as a core. Thus the Kohn et al. U.S. Pat. No. 3,325,037 and the Lippay U.S. Pat. No. 3,298,892 disclose structural sandwich laminates whose core is formed of end-grain balsa, the resultant laminates having a remarkably high strength-to-weight ratio as well as excellent thermal insulation properties.
End-grain balsa-core sandwich laminates are widely used in transportation and handling equipment, such as for floors of railroad cars, shipping containers, cargo pallets, bulkheads, doors and reefer bodies, as well as in a variety of other applications. These laminates are also employed for structural insulation in aircraft applications, in housing and in boating.
There are certain applications where the weight of existing types of laminates having a solid balsa core still exceeds optimal requirements despite the fact that balsa is inherently light weight. Thus in aircraft flooring or in radar domes constructed of balsa-wood sandwich laminates, it is desirable that the weight of the laminate be reduced without materially impairing the strength of the laminate.
An alternative core material for a laminate which affords reduced weight is a paper honeycomb constituted by an array of open cells having a square or other geometric configuration. A honeycomb of this type is extremely light but its structural properties are much inferior to those of a balsa core. Moreover since the facing skins must be laminated to the thin paper edges of the honeycomb, the bonding area is insufficient. To provide a strong and stiff sandwich structure, still another alternative lies in the use of a foam plastic core for a sandwhich laminate, but while such cores are very light in weight they possess relatively weak structural properties.
In the copending application Ser. No. 148,690 filed May 12, 1980 entitled Balsa-Core Sandwich Laminate, the entire disclosure of which is incorporated herein by reference, the core of a structural sandwich laminate is formed by a solid panel of end-grain balsa wood having holes punched therein to define a reticulated core of high strength and exceptionally light-weight.
While punching holes in a balsa panel results in some loss of strength, this loss is not proportional to the amount of wood removed therefrom. Thus, as pointed out in the copending application, one can by punching holes therein reduce the weight of an end-grain balsa core by 30 percent, yet retain 80% of its strength as compared to an unpunched panel. However, the material removed by punching is entirely wasted, and because Balsa is relatively expensive, this represents a significant economic loss.